Concern over energy utilization has resulted in the use of increasing amounts of insulation and higher R factors in building construction. Insulation of building walls to an R value of 35 and greater and insulation of roofs to an R value of 60 and greater is referred to as "superinsulation". A variety of building methods have been employed successfully for superinsulation of building walls. Similarly, in roofs with full attics superinsulation is commonly achieved by depositing, for example, fiberglass insulation to depths of 18 to 20 inches or more for achieving R factors of 70 and greater.
Superinsulation in the roof is difficult to achieve, however, for types of house and building construction which do not provide full attics in the roof. For example, cape style homes, saltboxes, roof dormers, hip roofs, gambrel, and cathedral ceiling designs may afford attic space only over a fraction of the roof area or no attic space at all. It is difficult and expensive to achieve superinsulation R values of 60 and greater in roofs of this type. The problem common to these types of roof construction is that the roof rafter is also the ceiling rafter thereby limiting the amount and depth of insulation to the size and depth of the rafter beam which in turn limits the depth of the rafter cavity.
The most frequently used method to achieve high R values in this type of roof construction is to use 2".times.12" rafters. Ten inches of fiberglass insulation is placed or blown in between the rafters with two inches of polyisocyanurate insulation on the inside face of the rafters. This is an expensive building technique which also presents problems with sheetrocking over the insulation board and placement of electrical fixtures. Furthermore, the insulation R factor falls short of superinsulation, for example R-48, and even this level of R value occurs inconsistently. Areas of higher heat conduction remain, caused by solid wood rafters which extend through the rafter cavity and increase the conduction losses. This conventional construction also results in "over building" with excessive volume of wood and excessive weight.
While the use of prefabricated roof trusses, for example, as described in U.S. Pat. No. 2,840,014, is well known in building construction, such prefabricated trusses suffer similar limitations in applications requiring increased insulation. Basically, only those prefabricated trusses providing a full attic space lend themselves to depositing or blowing in insulation to superinsulating depths. In applications, for example, to construction of capes, saltboxes, gambrel roofs, etc., prefabricated trusses do not lend themselves to enclosing and defining a high ceiling living space according to the desired style while at the same time accommodating depths of insulation for achieving very high R values. A typical handbook of truss configurations is found, for example, in the handbook Truss Shapes, "A Numerical System for Describing Truss Configurations", Technical Bulletin No. 2, Hydro-Air Engineering, Inc., 700 Office Parkway, Creve Coeur, Mo. 63141. Such conventional prefabricated roof trusses generally define an attic over a living space rather than enclosing the living space itself.